A headlight device needs to satisfy a predetermined light distribution pattern specified by road traffic rules or the like.
As one of the road traffic rules, for example, a predetermined light distribution pattern for an automobile low beam has a horizontally long shape narrow in an up-down direction. To prevent an oncoming vehicle from being dazzled, a boundary (cutoff line) of light on the upper side of the light distribution pattern is required to be sharp. A sharp cutoff line with a dark area above the cutoff line (outside the light distribution pattern) and a bright area below the cutoff line (inside the light distribution pattern) is required.
The illuminance is required to be highest at a region on the lower side of the cutoff line (inside the light distribution pattern). The region of highest illuminance is referred to as the “high illuminance region.” Here, “region on the lower side of the cutoff line” refers to an upper part of the light distribution pattern, and corresponds to a part for irradiating a distant area, in a headlight device. To achieve such a sharp cutoff line, large chromatic aberration, blur, or the like must not occur on the cutoff line. “Blur occurs on the cutoff line” indicates that the cutoff line is unclear.
To provide such a complicated light distribution pattern, an optical system configuration using the combination of a reflector, a light blocking plate, and a projection lens is commonly used (e.g., Patent Literature 1). The light blocking plate is disposed at a focal position of the projection lens.
In a headlight disclosed in Patent Literature 1, a semiconductor light source is disposed at a first focal point of a reflector with an ellipsoid of revolution. Light emitted from the semiconductor light source converges at a second focal point. The headlight disclosed in Patent Literature 1 blocks part of the light by a shade (light blocking plate) and then emits it through a projection lens ahead.